The invention relates to the automatic and continuous measurement of particles suspended in a liquid, and particularly to the measurement of liquids that flow in open channels or streams. This includes the effluents from waste treatment plants, settling tanks, ponds, streams, and lakes, and anywhere that low concentrations of solids are present along with adhesive scum, algae, and strong ambient light.
In such liquids it is desirable to measure the concentration of suspended solids continuously without disturbing the liquid to the extent that bottom sediment might be stirred up and mixed with an otherwise clear liquid or to the extent that delicate flocculated particles might be broken up or otherwide modified from their natural state prior to measurement. In addition, it is desirable to measure these relatively clear liquids without interference from ambient light (particularly sunlight) and without interference from scum or algea which can accumulate quite rapidly. These requirements inpose conflicting constraints on the design of an instrument and, up to the present time, no device has been available which satisfies all of them.
Furthermore, in the critical area of monitoring the effluents of municipal and industrial waste water treatment plants, any measurement errors or drift in calibration of a monitoring instrument have serious financial, legal, and environmental impacts which are so important that no prior instrument has been approved for continuous unattended monitoring. The problem here is not only that the instruments drift or change calibration, but that this drift goes on unnoted, and invalid measurements are indistinguishable from valid ones.
Aside from manual laboratory analysis, there are three methods that are suitable for measurement of suspended solids concentrations in relatively clear liquids: light scattering, light transmission, and a combination of the scattering and transmission. Methods that use light scattering either alone or in combination have high sensitivity but are subject to uneven response due to the widely varying scattering coefficients of many types of solids; they therefore are used successfully only in processes which have a singular type of solids. Light transmission instruments overcome this problem, but they have inherently lower sensitivity in low concentrations, unless very long light paths are used. Both types are affected by the presence of ambient light; in scattering instruments ambient light tends to give the same indication as an increase in solids concentration, and in transmission instruments ambient light produces the same indication as a decrease in solids concentration. Both types employ light shields or baffles to reduce ambient light, but since sunlight may be hundreds of times stronger than the instrument's light source, the shielding must be extremely effective, and most designs mount the optical detector in a pipeline, fed by a pump. The pipeline presents a tortuous path that greatly attenuates ambient light, but it also restricts the flow of the liquid. A pump then becomes necessary not only to guarantee that a constantly fresh sample is being measured, but also to provide some high velocity cleansing action to the measurement surfaces so that scum and algae do not build up and provide their own interference to the measurement (scum and algae both absorb and scatter light.) The pump and pipeline, no matter how small, tend to break up particles (particularly flocculated particles) into smaller particles, which in turn means that the character of the measured liquid is significantly altered from its original state. A pump also is an additional maintenance problem, since it can fail or clog with debris.